1. Field of the Invention
The present invention relates to improvements in replacement mechanical subassemblies and electronic enclosure units or panels such as prefabricated with bolt passage holes and designed to be bolted to a supporting structure having bolt receiving bores in predetermined fixed locations. The bolts may be threaded into floating nut plates attached to the support structure or into standard plain nuts. The aviation industry requires total interchangeability of such replacement units or line replaceable units (LRU's) of mechanical subassemblies and electronic enclosures for planes, space ships and other bodies. Such requirement presents alignment problems because the bolt receiving bores in the supporting structure and the bolt passage holes in the LRU's cannot be so precisely and immutably located, relative to each other, to provide exact registration or alignment with respect to all LRU's and all supporting structures. Center-to-center variations of up to about 0.03 inch are unavoidable and must be compensated.
One method for correcting for misalignment of bolt passage holes and bolt-receiving bores is to drill or re-drill and enlarge the bolt passage holes in the LRU on site for each assembly. Such a procedure is time-consuming and expensive and does not satisfy contractual requirements that line replacement units be prefabricated as interchangeable units.
Another method for compensating for small amounts of misalignment, up to about 0.03 inch, is to provide enlarged bolt passage holes or slots in the line replacement units so that although the bolt passage holes and the mounting structure bores are not concentric, the attachment bolts can be fastened into the bores and tightened to produce clamping frictional engagement between the LRU and the supporting structure.
However such frictional engagement does not provide protection against loosening and wear as a result of shear forces, such as is common in installations or assemblies subjected to vibration loads, as in the case of aircraft components. Vibrations produces shear forces, parallel to the mating surfaces of the LRU and the supporting structure, which can cause loosening of the bolts and relative slippage movement of the LRU and the supporting structure and enlargement or coining of the bolt passage holes and reduced reliability and life of the assembly. For these reasons the use of enlarged or slotted bolt passage holes, or any other system which does not provide shear force protection, is not permitted in critical assemblies which are subjected to high shear forces, such as assemblies subjected to substantial vibration forces.
Ideally, the shank of the attachment bolts will contact both the wall of the bolt passage holes in the LRU and the bolt passage holes in the support structure when the bolts are tightened into the threaded nut plates attached to the supporting structure so as to provide resistance to shear forces independent of frictional clamping engagement. Such engagement resists up to about fifteen times as much force as compared to assemblies using oversize bolt passage holes, the walls of which do not contact the shank of the tightened attachment bolts. As mentioned, the use of small or close tolerance bolt passage holes provides the desired bolt hole passage wall engagement but results in the misalignment problems discussed supra.
2. Discussion of the Prior Art
Reference is made to U.S. Pat. No. 3,006,443 which discloses an assembly for compensating for or correcting misalignment of the bolt passage holes in two sheets or plates being fastened together by means of bolts and nuts. Such assemblies are only useful for installations providing interior access for the attachment of bolt engaging nuts. Moreover such assemblies require the production, placement and manual positioning and orientation of bolt hole passage members in each of the sheets or plates being fastened together so that the bolted assemblies provide resistance against shear forces. Thus, the objectives and results of the invention of U.S. Pat. No. 3,006,443 are similar to those of the present invention but the means for producing said results differ substantially from those of the present invention.
Reference is also made to U.S. Pat. No. 3,382,630 which discloses nut-and-bolt assemblies for providing tiltable motion or self-adjusting connections between transparent windshield panels and supporting structure on an aircraft windscreen. Shear resistance is not provided since the connecting bolts do not maintain contact with the walls of the hole passages. Reference is also made to U.S. Pat. Nos. 3,145,362 and 4,860,513 which disclose fastening bolts having heads with rounded or tapered undersurfaces for engaging mating concave or tapered surfaces on a panel designed to be fastened to a supporting surface. The functions of these assemblies do not involve misalignment compensation or shear force resistance.